The invention relates to an electronic power meter using a processor for all its computations and includes compensation of phase lag on the current transformers used for sensing the line currents.
Conventional power meters utilize current transformers for sensing line currents. As inductive elements, all current transformers exhibit definite phase lag between the primary and the secondary. The error due to this phase lag affects the accuracy of measurements and thereby resulting in overall inaccuracy. Power meter manufacturers are used to implementing conventional phase shifters using variable resistors and/or variable capacitors for each of the three phases increasing the material and production cost. Moreover, variable RC network phase shifting is not applicable to multiplexed signals in three phase electronic power meters. Phase compensation techniques have been previously described, for example, in U.S. Pat. Nos. 5,017,860 and 5,231,347.
Techniques have also been developed that use two analog-to-digital converters and by shifting the sampling time of one of the external circuitry increasing the cost of the hardware. These techniques do not compensate for the non-linearity in the phase shift with respect to the current that exists on the current transformers.
It is therefore an object of the invention to provide a simplified phase adjustment for an electronic power meter. It is another object of the invention to provide the exact amount of phased shift required for each of the current transformers on each of the phases, which could be different. It is yet another object of the invention to provide different amount of phase compensation at different currents to compensate for the non-linearity on the current transformer.
In practicing the invention, an electronic solid state power meter is provided for single phase power line and polyphase power lines utilizing a multi-channel analog to digital converter in built in the processor, to first provide a phase compensation on the current and voltage signals using time shifted averaging technique implemented in software and then subsequently using this compensated data for computing the powers and energies. The invention is very useful and is not computational intensive technique.
In accordance with the invention, a processor compensates the phase shift on current transformers by first estimating the delay between two samples of current signal for subsequent averaging in computations in software. Estimating the amount of desired variation of delay with respect to current will not only compensate for the phase shift on the current transformers, but also compensates for the non-linearity in the phase shift. The phase voltages are scaled down to lower voltages using potential dividers and currents are fed to the primary of the current transformers. The secondary of the current transformer provides a current output proportional to turn ratio between the secondary and the primary of the transformer. A resistor with very low temperature coefficient terminated on the secondary of the current transformer provides a potential proportional to the value of the resistance and the current in the secondary. These signal conditioned phase voltage and line current signals are fed to the analog to digital converter, which is part of the processor, to digitize at periodic intervals. The digitized signal are used by the processor to multiply and compute instantaneous powers and are then integrated for a finite number of mains cycles to compute energy. The computed energy value is stored in the internal non-volatile memory at desired intervals of time.